An image forming apparatus of an electrophotographic system such as a laser printer, a digital copying machine, or a laser facsimile includes an optical beam scanning apparatus that forms an electrostatic latent image on a photoconductive drum by irradiating a laser beam (a luminous flux) on the surface of a photoconductive drum and scanning the laser beam.
Recently, besides a monochrome machine including a scanning optical system that uses a single light source, a tandem color machine is proposed. For the tandem color machine, a method of increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) in one laser unit (a multi-beam method) is proposed for the purpose of realizing an increase in speed of scanning on the surface of a photoconductive drum. In the multi-beam method, plural beams for each of color components (e.g., yellow, magenta, cyan, and black) emitted from the respective light sources are subjected to processing in a pre-deflection optical system and are changed to one beam and made incident on a polygon mirror. The beam deflected by the polygon mirror is, after passing through an fθ lens configuring a post-deflection optical system, separated into beams for each of the color components and irradiated on a photoconductive drum for each of the color components.
There is also proposed a color image forming apparatus including an optical beam scanning apparatus that forms plural scanning lines by separating one or plural luminous fluxes, which are emitted from one or plural light sources, for each of color components using plural deflection surfaces having different angles with respect to a rotation center axis of a deflecting device (a polygon mirror) and then, imaging the luminous fluxes with a post-deflection optical system. For example, according to JP-A-7-256926, there is known a technique for making plural beams incident on an identical surface of one polygon mirror, separating the beams in order of height in a sub-scanning direction after the beams pass through one set of fθ lenses, and reflecting the separated beams.
Moreover, there is also recently proposed an optical beam scanning apparatus in which an individual focusing lens is provided for each of color components in a post-deflection optical system in order to improve optical accuracy in a scanning optical system in the optical beam scanning apparatus. For example, according to JP-A-2003-5113, there is known a technique for making plural beams on an identical surface of one polygon mirror, separating the beams in order of height in a sub-scanning direction after the beams pass through a shared fθ lens, and making the separated beams incident on another fθ lens after reflecting the beams.
Conventionally, in the optical beam scanning apparatus in which the individual focusing lens is provided for each of the color components in the post-deflection optical system, positions of individual focusing lenses included in the post-deflection optical system and other optical elements are fixed. Therefore, when defocus in a main scanning direction or a sub-scanning direction occurs with respect to optical elements including the individual focusing lenses provided in the post-deflection optical system because of an error during manufacturing of a housing, the optical elements including the individual focusing lenses provided in the post-deflection optical system cannot be adjusted and it is difficult to obtain optical characteristics as designed.